Peter Hermann Stillmark (22 July 1860, Penza, Russian Empire – 23 June 1923, Pärnu, Estonia) was a Baltic-German microbiologist.

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In 1888 at the University in Dorpat, now Tartu in Estonia under Professor Rudolf Kobert's supervision, he completed his doctoral thesis Über Ricin, ein giftiges Ferment aus den Samen von Ricinus comm. L. und einigen anderen Euphorbiaceen, which is a description of the isolation of ricin, a poisonous protein component from castor beans.^[1][2] That event is internationally recognized as the discovery of a class of carbohydrate-binding proteins called lectins and the birth of a new branch of science called lectinology.

In 1988 the University of Tartu crafted a medal to commemorate the centennial of Stillmark’s discovery. It was awarded to Samuel Barondes in September 1990 at the 12th meeting of the International Lectin Society (Interlec-12) in Davis California for his pioneering studies of the multiple biological functions of galectins, a large family of lectins found in many animals, including humans.